Printing plates suitable for imaging by digitally controlled laser radiation are known in the prior art. However, none of the prior art printing plates is entirely suitable for its intended purpose. Accordingly, there remains a need for a printing plate suitable for laser imaging that is both effective and economical.
Laser radiation suitable for imaging printing plates preferably has a wavelength in the near-infrared region, between about 700 and 1500 nm. Solid state laser sources (commonly termed "semiconductor lasers") are economical and convenient sources that may be used with a variety of imaging devices. Other laser sources such as CO.sub.2 lasers and lasers emitting light in the visible and UV wavelengths are also useful.
Laser output can be provided directly to the plate surface via lenses or other beam-guiding components, or transmitted to the surface of a blank printing plate from a remotely sited laser through a fiber-optic cable. A controller and associated positioning hardware maintains the beam output at a precise orientation with respect to the plate surface, scans the output over the surface, and activates the laser at positions adjacent selected points or areas of the plate. The controller responds to incoming image signals corresponding to the original figure or document being copied onto the plate to produce a precise negative or positive image of that original. The image signals are stored as a bitmap data file on a computer. Such files may be generated by a raster image processor (RIP) or other suitable means. For example, a RIP can accept data in page-description language, which defines all of the features required to be transferred onto a printing plate, or as a combination of page-description language and one or more image data files. The bitmaps are constructed to define the hue of the color as well as screen frequencies and angles.
The imaging apparatus can operate on its own, functioning solely as a platemaker, or can be incorporated directly into a lithographic printing press. In the latter case, printing may commence immediately after application of the image to a blank plate, thereby reducing press set-up time considerably. The imaging apparatus can be configured as a flatbed recorder or as a drum recorder, with the lithographic plate blank mounted to the interior or exterior cylindrical surface of the drum. Obviously, the exterior drum design is more appropriate to use in situ, on a lithographic press, in which case the print cylinder itself constitutes the drum component of the recorder or plotter.
In the drum configuration, the requisite relative motion between the laser beam and the plate is achieved by rotating the drum (and the plate mounted thereon) about its axis and moving the beam parallel to the rotation axis, thereby scanning the plate circumferentially so the image "grows" in the axial direction. Alternatively, the beam can move parallel to the drum axis and, after each pass across the plate, increment angularly so that the image on the plate "grows" circumferentially. In both cases, after a complete scan by the beam, an image corresponding (positively or negatively) to the original document or picture will have been applied to the surface of the plate.
In the flatbed configuration, the beam is drawn across either axis of the plate, and is indexed along the other axis after each pass. Of course, the requisite relative motion between the beam and the plate may be produced by movement of the plate rather than (or in addition to) movement of the beam.
Regardless of the manner in which the beam is scanned, it is generally preferable (for reasons of speed) to employ a plurality of lasers and guide their outputs to a single writing array. The writing array is then indexed, after completion of each pass across or along the plate, a distance determined by the number of beams emanating from the array, and by the desired resolution (i.e., the number of image points per unit length).
Some prior art patents disclosing printing plates suitable for imaging by laser ablation are Lewis et al U.S. Pat. Nos. 5,339,737 and 5,353,705 and Nowak et al Re. U.S. Pat. No. 35,512. The disclosures of those patents are incorporated herein, to the extent consistent with our invention.
Although these prior art printing plates perform adequately, they are expensive to produce because the absorbing layer is vapor deposited onto the oleophilic polyester layer. Adhesive bonding of the polyester layer to a metal substrate also adds to the cost.
A principal objective of the present invention is to provide a printing plate suitable for imaging by laser radiation wherein the absorbing layer is a plurality of particles or a thermoplastic or elastomeric layer.
A related objective of the invention is to provide a printing plate suitable for imaging by laser radiation wherein a thermoplastic or elastomeric layer is joined to a metal substrate without an additional adhesive layer between the thermoplastic or elastomeric layer and the metal substrate.
Further objectives and advantages of our invention will become apparent to persons skilled in the art from the following detailed description.